As a conventional stay for opening and closing of a door which facilitates an opening and closing operation of the door or a cover, there is known a stay having a slider crank mechanism 5 inside as illustrated in FIGS. 12(a) and 12(b) (see PL 1). A main body 1 of the stay is rotatably connected to an end 2a of an arm 2. The main body 1 is mounted on a housing 6 and the arm 2 is connected to the door 7. A force for assisting the opening and closing operation is transmitted to the door 7 from the arm 2.
The main body 1 is provided with a coil spring 8. An end of a link 3 is rotatably connected to a main-body slider 4 biased downward by the coil spring 8. The other end of the link 3 is connected to a pivot base part of the arm 2. These main-body slider 4, link and arm 2 form the slider crank mechanism 5. When the main-body slider 4 moves back and forth, the arm 2 equivalent to a crank rotates via the link 3. Amounting seat 10 is rotatably mounted to the other end 2b of the arm 2 via another link 9. The door 7 is attached to this mounting seat 10.
As illustrated in FIG. 12(a), when the door 7 is closed, a torque T1 acts on the arm. 2 in a clockwise direction by a biasing force of the coil spring 8. Thus, as the torque in the closing direction still acts on the closed door 7, the closed state of the door can be kept stable. Meanwhile, as illustrated in FIG. 12(b), when opening the closed door 7, the arm 2 rotates and the slider crank mechanism 5 goes beyond a change point. Then, a torque T2 acts on the arm 2 in a counterclockwise direction by the biasing force of the coil spring 8. As the door 7 is further acted upon by the torque in the opening direction, the opening operation of the door 7 can be facilitated and the open angle of the door can be kept constant.